Photogrammetry is the science of determining dimensions of objects revealed by photographic images. Relative dimensions of the objects are derived from the photographs and are scaled, usually as a photogrammetric machine function, with reference to actual measurements which are obtained, separately from the photographs, of one or more points in the field of the photographs. The science has developed since the 1920's and, since about 1940, particular emphasis has been placed on the production of high quality topographical maps using stereoscopic aerial photography.
Apparatus which currently is being employed for photogrammetric measurement may be regarded as falling in one of two general groups.
The first group includes Mechanical Analogue Machines which are operated by setting a stereo-pair of photographs in their correct relative orientation and by arranging a viewing system such that mechanical "observing rays" are coupled to a drawing mechanism.
The second group includes so-called Analytical Machines which measure the co-ordinates of selected corresponding points on each photograph of a stereo-pair and compute from the derived observations the true position of each point. These machines may be categorized as falling within one of two sub-groups, consisting of Simple Systems and Precision Analytical Machines. In the Simple Systems an operator manually positions a reference mark on a selected point on each photograph of a stereo-pair, obtains a read-out of ordinates with respect to the photo-centre and computes from this data the actual geographic location of a point. The Precision Analytical Machines are highly complex machines which incorporate built-in computers and elaborate servomechanisms for the purpose of providing an opto-electrical function which is analogous to the Mechanical Analogue Machines.
Of the abovementioned known types of apparatus, the Mechanical Analogue Machines are expensive, are difficult to set-up (the positioning of the photographs is generally effected by an iterative process that takes from one to six hours per model), and require a highly trained operator. Moreover, with the Mechanical Analogue Machines, the focal point is fixed mechanically in the plotter and the machines can only be used with photographs which have been obtained from compatible cameras. Additionally, corrections for problems such as lens distortion in the camera have to be made by use of elaborately constructed optical or mechanical correcting devices. The known Analytical Machines avoid most of these problems, but the simple types are slow to use and relatively inaccurate and the Precision Machines are very expensive.
The abovementioned types of apparatus are suitable for use by mapping organisations that can justify heavy capital expenditure and specialized staff employment. However, there are situations, such as in the mining or forestry industries or in military survey applications, which require relatively inexpensive photogrammetry machines which can be set up quickly and simply by an operator with limited photogrammetric training.